The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Heat is required in many applications including power generation and chemical processing. The sources of heat are diverse but the dominant source is presently fossil fuels that produce significant amounts of carbon dioxide (CO2). This heat is typically collected in boilers, for power generation, or furnaces, for chemical processing. While biomass and waste fuels can partially displace the use of fossil fuels, they are a limited resource and can only substitute a fraction of the need. The leading alternative renewable source of high temperature heat is concentrated solar thermal energy, which is collected in a solar receiver, from mirrors used to concentrate the direct component of solar radiation.
Power generation using concentrating solar thermal technologies remains significantly more expensive than conventional, fossil fuel power. In addition, while solar thermal power allows for thermal storage, it is not cost-effective to provide sufficient storage for periods of extended cloud cover. Despite its advantages, storage also adds to capital cost, which is a further barrier to implementation.
Hybrid solar and combustion systems can avoid the need for thermal storage, which is otherwise required in solar-only systems during periods of low insolation. The state-of-the-art in hybrid systems co-locate solar receivers, which are designed as stand alone systems, with combustors that are designed solely for combustion. Such a system typically results in the solar system cooling down during the night and the combustor being turned down (at least partly) during the day. This results in significant heat losses and inefficiency. It also requires heat exchangers for each energy source, adding to the capital cost of the combined system. The percentage of renewable energy that can be achieved by solar-contribution hybrid systems without storage is also limited.
It is desirable for a hybrid solar and combustion system to be operable continuously for extended periods of time and to also be capable of mitigating against thermal shock arising from rapid variations in the intensity of available solar radiation. A rapid change in solar flux may, for example, be induced by the passage of clouds over a solar concentrator (such as a heliostat field).
There are, however, various problems and difficulties that confront known hybrid solar and combustion systems in relation to operation for extended periods of time and also thermal shock, some of which are outlined in more detail below.
An example of a hybrid solar and combustion system which co-locates the solar receiver and the combustor is disclosed in U.S. Pat. No. 4,602,614 (Percival et al). Specifically, the disclosure concerns a device which includes a receiver cavity, a heat exchanger within the receiver cavity to receive heat from a combustion process within the receiver cavity, and an aperture for admitting solar radiation into the receiver cavity to impinge upon the heat exchanger. The aperture incorporates a window for preventing the escape of combustion gases from the cavity during the combustion process. With this arrangement, the window provides a physical barrier that must be maintained in order for the hybrid solar and combustion system to operate effectively. However, the window is of a configuration which intrudes into the device, thereby limiting the available volume to accommodate the cavity. Moreover, the presence of a window is problematic as it is increasingly vulnerable to damage as the concentration ratio of the concentrated solar radiation is increased. At high fluxes, even small particles can cause damage to the surface of the window and lead to a high risk of failure. Further, it is necessary to maintain the window in a relatively clean condition in order for the system to operate effectively using solar radiation. This can add significantly to operating and maintenance costs. Still further, the use of the window requires that operation of the hybrid solar and combustion system be interrupted to clean the window, thereby preventing continuous, long-term operation of the system using both combustion and solar energy sources.
A further example of a hybrid solar and combustion system which co-locates the solar receiver and the combustor is disclosed in U.S. Pat. No. 5,884,481 (Johansson et al) which provides a device having a receiver chamber, a heat exchanger within the receiver chamber, and an aperture for admitting solar radiation into the receiver chamber to impinge upon the heat exchanger. The device also has facility for a combustion process to produce combustion gases which contact the heat exchanger. In the embodiment described and illustrated in U.S. Pat. No. 5,884,481, an aperture cover is provided for selectively opening and closing the aperture. The embodiment has two operating modes. One mode is a solar operating mode in which the aperture cover is moved away from the aperture to permit solar radiation to insolate the receiver chamber. The other mode is a combustion mode in which the aperture cover is moved into sealing contact with the aperture. With this arrangement, the device can operate only in the solar operating mode or the combustion mode, according to the position of the aperture cover. In another embodiment, which is described but not illustrated, the receiver aperture is covered with a transparent cover, such as a quartz lens, which allows solar energy to enter the receiver chamber but prevents heated fluid from escaping from the receiver chamber. While this arrangement would allow the system to operate in both modes at the same time, it would almost certainly experience the problems referred to above associated with the window in U.S. Pat. No. 4,602,614.
A further problem likely to be encountered with the arrangement disclosed in U.S. Pat. No. 5,884,481, when in the solar operating mode, is that of thermal shock arising from rapid variations in the intensity of available solar radiation. A rapid change in heat-flux can generate high stresses in the heat exchange materials, the management of which can incur significant penalties. By way of example, the use of more exotic, high temperature materials would almost certainly increase costs.
A still further example of a hybrid solar and combustion system which co-locates the solar receiver and the combustor is disclosed in US 2002/0059798 (Mehos et al). Specifically, the disclosure concerns a device which includes a front dome presenting a solar absorber surface which is exposed to solar radiation entering the dome. The device also has a combustion system separate from the front dome. In other words, the device has separate zones for the collection of solar energy and combustion energy. With this arrangement there is no requirement for a window to facilitate entry of solar radiation while inhibiting combustion heat losses. However, because the device has separate zones for the collection of solar energy and combustion energy, it is likely that the device will need to be relatively large and will also be prone to higher heat losses. Furthermore, the device does not have the capacity to mitigate against thermal shock.
It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.
Accordingly, it is an object of the invention to provide a hybrid receiver-combustor that solves or ameliorates the abovementioned problems, or at least offers a useful choice.